/opt/local/var/macports

[Joshua Root]

> Are there negative side-effects to moving this largish subdirectory to an external drive and connecting via soft link?

I haven't tried it myself in a while, but as far as I know having it on 
another volume should work fine. You don't need the link, since this 
location is configurable as 'portdbpath' in macports.conf. The negative 
effects are really just the usual effects of using an external drive. 
Builds and registry operations will be limited by the drive's speed, and 
there will be an additional performance penalty when activating ports 
because the files are extracted in the software directory and then moved 
into place, and that move will be across volumes with this configuration.

- Josh